A recent development in the recovery of upgraded oil products from surface-mined oil sands located in the Fort McMurray region involves the formation of a low-temperature, deaerated bitumen froth at locations that may be far removed from the upgrading facilities. Hence, the bitumen froth may need to be pumped through a pipeline over long distances (in order of 35 km) so that the froth can be further upgraded at the existing upgrading facilities.
The bitumen froth that is produced from oil sands routinely contains about 20-40% by volume dispersed water in which colloidal clay particles are well dispersed. Such an oil-water mixture is very stable and very viscous, having viscosities even higher than the oil alone.
It can be very costly from an energy standpoint to transport a viscous material such as bitumen froth through a pipeline. Significant pressure drops can occur along the pipeline due to the shear stresses between the pipe wall and the viscous fluid. Also, the oily, viscous fluid being transported may cause "fouling" of the pipeline to occur. Fouling of the pipeline is a result of oil sticking to the generally oleophilic pipe walls, particularly at sites of sharp changes in flow direction, thereby resulting in a continual increase in the pressure gradient required to drive the flow. The fouling may ultimately result in the total blockage of the pipeline.
A known procedure for reducing some of the aforementioned problems encountered in transporting viscous oils through a pipeline involves the introduction of a less viscous immiscible fluid such as water into the flow of oil, to act as a lubricating layer between the pipe wall and the oil. This procedure for transporting viscous oil is commonly referred to as core-annular flow.
The conventional means for establishing core-annular flow is to inject water and oil simultaneously, with the water collecting in the annulus and encapsulating the oil core.
The design of injection nozzles and control of the flow rates impacts on the formation of a lubricated layer and on the time and downstream distance necessary to establish lubricated flow. Establishing lubricated flow in conventional applications is a manageable problem that can usually be controlled by varying the rate of water and oil injection. In fact, different flow types with different pressure gradients can be achieved by varying the injection rates (see, for example, Joseph, D. D. & Renardy, Y. Y., (1992), Fundamentals of Two-Fluid Dynamics, Part II: Lubricated Transport, Drops and Miscible Liquids. (Springer, N.Y.)).
Conventional methods for establishing core-annular flow are impractical for the start-up of core flow of bitumen froth, as the addition of water is undesirable. As previously stated, the froth already contains 20-40% water by volume in its natural state and therefore, the addition of more water makes the separation of bitumen from water in subsequent processing more difficult. In addition, adding more water will decompose the froth. Hence, it was necessary to develop a process where core-annular flow could be achieved in the pipeline without requiring the addition of more water to the froth. This invention is directed towards a process that allows the bitumen froth to be self-lubricating. In other words, it is the water already present in the bitumen froth that forms the lubricating outer layer surrounding the oily core.
A method for starting self-lubricated flow of water in oil emulsions (5 to 60% water by weight) was described in the U.S. Pat. No. 4,047,539 to Kruka. This patent teaches the start-up of self-lubricated flow of emulsions of water in Midway-Sunset crude oils by creating a certain shear rate for a certain length of time in a pipe flow to break the emulsion and create a water rich zone near the pipe wall. When the water in oil emulsions are subjected to faster shearing, water droplets are produced and these water droplets will tend to coalesce and form a self-lubricating layer of free froth water. The shear rates required to break up the emulsion were achieved by slow increases in pressure. However, the method of slow increases in pressure will not work in long commercial pipelines because the pressure drop required. to produce the critical shear rates is too large. Hence, the Kruka process can not be used for the transport of bitumen froth through 35 km of pipeline.
A process for restarting core flow of viscous oils after a long standstill period was described in U.S. Pat. No. 4,753,261 to Zagustin et al. The process involved the controlled injection of water. However, this process still requires the injection of more water than is desirable when attempting to start the self-lubricated flow of bitumen froth.